Quantifying the importance of authigenic carbonate in the ancient carbon cycle

在地球历史研究中，由于碳作为行星恒温器和大气氧气来源的双重作用，可以说没有任何元素比碳受到更多的关注。由于直接量化质量通量的挑战，特别是在深部，沉积相的碳同位素测量(13C)已成为研究全球碳循环的代用品。然而，最近的工作挑战了用于解释13C记录的传统范式，提出了自生碳酸盐是在厌氧有机物降解过程中释放的碳在数量上是一个重要的第三碳汇--特别是在氧气浓度较低的时期，如前寒武纪和显生宙的短暂缺氧期间。 如果这个模型是正确的，那么我们对碳循环和全球氧化还原预算的理解需要从根本上得到纠正。然而，测试这一模型具有挑战性，因为自生碳酸盐可以散布在大量埋藏的硅屑沉积物中。因此，这些方法必然是互补的，而不是将地球化学指标视为地质质量通量经验量化的替代品。例如，来自北美上地壳4D模型Macrostrat的分析揭示了泥盆纪末陆架碳酸盐生产的崩溃，取而代之的是硅碎屑。据预测，这一情景将为自生碳酸盐沉没在另一个有氧世界中的重新出现创造理想的边界条件。 这项建议概述了一种独特和创新的方法组合--实地工作、同位素地球化学、计算能力--以评估深部自生水平的强度。野外工作既要寻找泥盆纪末的碳酸盐，又要约束海水化学成分，这是对该模型的一级检验。在加拿大前缘山脉，泥盆纪末期和早石炭世地层厚达数千米。工作将包括同时记录地层剖面和对平台碳酸盐及其盆地等价物进行地球化学采样。收集的样本将在维多利亚大学进行13C和微量元素丰度分析。开发的地球化学数据将被用来模拟自生碳酸盐的重要性，方法是将这些沉积物成分的测量与Macrostrat得出的沉积质量通量估计相结合。这些努力将利用新产生的数据和使用机器阅读和数据挖掘技术从文献中提取的数据(Geodeepdive.org)。不可避免地，这种计算方法将揭示数据差距并提出新的假设，从而为进一步的实地和实验室工作提供新的方向和动力。

In Earth history research, arguably no element has received more attention than carbon, owing to its dual role as a planetary thermostat and source of atmospheric oxygen. Due to the challenges of direct quantification of mass fluxes, especially in deep time, carbon isotopic measurements (13C) of sedimentary phases have emerged as proxies to study the global carbon cycle. Recent work, however, has challenged the traditional paradigm used to interpret 13C records by suggesting that authigenic carbonate, precipitated from carbon released during anaerobic organic matter degradation, is a quantitatively important third sink for carbon - especially during times of putatively lower O2 concentrations, such as during the Precambrian and transient anoxia in the Phanerozoic. If this model is correct, then our understanding of the carbon cycle and global redox budgets needs to be redressed at a fundamental level. Testing this model is challenging, however, because authigenic carbonate can be disseminated over a large volume of buried siliciclastic sediment. Thus, rather than viewing geochemical proxies as replacements for empirical quantification of geological mass fluxes, these approaches are necessarily complementary. For example, analyses from Macrostrat, a 4D model of the North American upper crust (macrostrat.org), reveal an end-Devonian crash in shelf carbonate production, with replacement by siliciclastics. This scenario is predicted to produce ideal boundary conditions for the reemergence of an authigenic carbonate sink in an otherwise oxic world. This proposal outlines a unique and innovative combination of approaches - field work, isotope geochemistry, computational abilities - to assess the strength of the authigenic lever in deep time. Field work that both searches for these end-Devonian carbonates and constrains the composition of seawater chemistry is a first-order test of the model. In the Canadian Front Ranges, end Devonian and early Carboniferous strata are kilometers thick. Work will involve concurrent logging of stratigraphic sections and geochemical sampling of both platformal carbonates and their basinal equivalents. Collected samples will be analyzed for 13C and trace element abundances at the University of Victoria. The geochemical data developed will be used to model the importance of authigenic carbonate by combining these measurements of sediment composition with the sedimentary mass flux estimates derived from Macrostrat. These efforts will utilize both the newly produced data and data extracted from the literature using machine reading and data mining techniques (geodeepdive.org). Inevitably, this computational approach will reveal data gaps and pose new hypotheses, thereby providing new directions and motivation for further field and laboratory work.